Task Plus 

Task Plus is a very graphical freeware task and 4-calendar scheduler designed for simplicity and speed. Unlimited number of personal tasks on the calendar with holidays and repeating events such as birthdays. 4 month calendar is color coded.
calendar appointment scheduler alarms popup alarms pim

TrayDay 

TrayDay: today's date at a glance, in the Windows tray, with handy popup calendar. Can show week numbers to ISO (or your own) standard. A double-click types the date in the format of your choice. Julian Day and Internet Time features.
calendar date internet time swatch julian tray counter millennium week number

MicroCalendar - Windows Tray Calendar 

MicroCalendar is a Windows system tray popup calendar. It is small, unobtrusive, and always ready so you can easily check calendar dates without messing up your computer's system time or having to use a resource-intensive application.
tray calendar microcalendarr pop-up calendar system tray simple calendar calendar